About one third of all sports injuries admitted to outpatient sports clinics concern knees. The incidence of different knee disorders in an outpatient sports clinic material and a review of the literature concerning the wide and problematic area of knee exertion injuries in athletes is presented. Exertion injury is defined as non-traumatic pain syndrome in the musculoskeletal system, including typical stress injuries and pain syndromes associated with physical activity but also has other important aetiological factors. 2762 athletes were admitted to the Turku Sports Medical Research Unit's outpatient sports clinic from 1976 to 1983 and 886 (32%) of them, 697 male and 189 female athletes, had suffered knee disorders. Football (soccer) [20.8%], long-distance running (13.1%), volleyball (11.6%), orienteering (7.6%) and ice-hockey (7.2%) had the highest incidence and the most common knee disorders were patellar apicitis (20.4%), Osgood-Schlatter's disease (10.1%), patellar chondropathy (10.0%), ligamentous sprains (9.0%) and meniscus tears (6.9%). The mean age of all athletes with knee disorders was 20.8 years. On an average each complaint caused 2.16 appointments. Careful evaluation of malalignments predisposing the athlete to exertion injury is necessary in the treatment of knee disorders and to avoid the recurrence of the exertion injury due to some biomechanical reason. The authors emphasise the importance of careful clinical examination. Although our review does not include detailed information about injuries originating in a single trauma, it is important to pick up the cases of ligamentous tears early so they can be appropriately repaired. Diagnostic and operative arthroscopy adds a new method in avoiding diagnostic errors and in shortening the postoperative rehabilitation.